In known types of hydrofoil seacraft, the hull of the craft is lifted out of the water by means of foils carried on struts which pass through the water beneath the surface thereof. In passing through the water, and assuming that sufficient speed is attained, the foils create enough lift to raise the hull above the surface and, hence, eliminate the normal resistance encountered by a ship hull in passing through the water.
In the usual case, there are forward and aft foils, both provided with control flaps similar to those used on aircraft. The other essential element is the rudder which pierces or is submerged beneath the surface of the water and is either forward or aft of the craft, depending upon its design. In most hydrofoils, the flaps are used primarily to cause the craft to ascend or descend and to control the craft about its pitch and roll axes; however they are also used in combination with the rudder to bank the craft about its roll axis during a turn. A typical control system for hydrofoils of this type is shown in U.S. Pat. No. 3,886,884, issued June 3, 1975 and assigned to the Assignee of the present application.
In any hydrofoil control system, safety is a paramount factor to be taken into consideration. In this respect, provision should be made for causing the craft to descend from its foil-borne to its hull-borne mode of operation quickly and automatically in the event of a primary electronic power source failure or some other off-normal condition before an unsafe roll or yaw attitude is developed while the craft is foil-borne with its hull elevated above the water surface.
In U.S. Pat. Nos. 3,800,727, issued Apr. 2, 1974, and also assigned to the Assignee of the present application, such a system is described for automatically initiating a transition from the foil-borne to the hull-borne mode of operation of a hydrofoil craft upon the occurrence of a power failure or other off-normal condition. The invention described in U.S. Pat. No. 3,800,727 provides for two separate servo systems for controlling the rudder and each of the flaps on the foils or control surfaces of a hydrofoil craft. One servo system for each control surface is powered by the main power supply for the craft, usually an alternating current power source; while the other system is powered by a separate source, usually batteries providing direct current. During normal operation of the craft, both the alternating current servo system and the alternate direct current powered servo system are connected to the flap actuators. However, control of the control surface positions is dominated by the alternating current system which causes the control surfaces to position in response to commands from the pilothouse as well as motion sensing devices. But upon a failure of the main control system, the direct current auxiliary servo system, which provides a small control command at all times, takes over and causes the control surfaces to move to predetermined positions which will force the craft to land rapidly. Since the auxiliary feedback null positions for an automatic landing, no landing commands are necessary from the pilothouse.
While the system shown in U.S. Pat. No. 3,800,727 operates entirely satisfactorily while the craft is in straight-away operation at the instant of a primary power failure, if the power failure should occur during a turning maneuver, the craft attitudes and rates and the forces resulting from impacting the water are very severe, even for very moderate initial turn rates.